The importance of potassium channels was first recognized aproximately fifty years ago when Hodgkin and Huxley discovered that potassium ions contributed to the current that excited the squid giant axon. Research in the area, however, was hampered by the lack of selective, high affinity ligands for potassium channels. But the advent of recombinant DNA techniques and single cell and whole cell voltage clamp techniques has changed the slow pace of the field. Indeed, potassium channels that exhibit functional, pharmacological and tissue distribution characteristics have been cloned. These cloned potassim channels are useful targets in assays for identifying candidate compounds for the treatment of various disease states. Potassium channels have turned out to be the most diverse family of ion channels discovered to date. They modulate a number of cellular events such as muscle contraction, neuro-endocrine secretion, frequency and duration of action potentials, electrolyte homeostatis, and resting membrane potential.
Potassium channels are expressed in eukaryotic and procaryotic cells and are elements in the control of electrical and non-electrical cellular functions. Potassium channels have been classified according to their biophysical and pharmacological characteristics. Subclasses of these channels have been named based on amino acid sequence and functional properties. Salient among these are the voltage dependent potassium channels, for example voltage gated potassium channels (e.g., Kv1, Kv2, Kv3, Kv4). Subtypes within these subclasses have been characterized as to their putative function, pharmacology and distribution in cells and tissues (Chandy and Gutman, “Voltage-gated potassium channel genes” in Handbook of Receptors and Channels—Ligand and Voltage-gated Ion Channels, ed. R. A. North, 1995; Doupnik et al., Curr. Opin. Neurobiol. 5:268, 1995). For example, the Kv1 class of potassium channels is further subdivided depending on the molecular sequence of the channel, for example Kv1.1, Kv1.2, Kv1.3, Kv1.4, Kv1.5, Kv1.6, and Kv1.7. Functional voltage-gated K+ channels can exist as multimeric structures formed by the association of either identical or dissimilar subunits. This phenomena is thought to account for the wide diversity of K+ channels. However, subunit compositions of native K+ channels and the physiologic role that particular channels play are, in most cases, still unclear.
Membrane depolarization by Kv1.3 inhibition has been shown to be an effective method to prevent T-cell proliferation and therefore has applications in many autoimmune conditions. Inhibition of K+ channels in the plasma membrane of human T-lymphocytes has been postulated to play a role in eliciting immunosuppressive responses by regulating intracellular Ca++ homeostasis, which has been found to be important in T-cell activation.
The Kv1.3 voltage-gated potassium channel is found in neurons, blood cells, osteoclasts and T-lymphocytes. The Chandy and Cahalan laboratories proposed a hypothesis that blocking the Kv1.3 channel would elicit an immunosuppressant response. (Chandy et al., J. Exp. Med. 160, 369, 1984; Decoursey et al., Nature, 307, 465, 1984). However, the K+ channel blockers employed in their studies were non-selective. Until research with the peptide margatoxin, a peptide found in scorpion venom, no specific inhibitor of the Kv1.3 channel existed to test this hypothesis. Although a laboratory (Price et al., Proc. Natl, Acad, Sci. USA, 86, 10171, 1989) showed that charybdotoxin would block Kv1.3 in human T-cells, charybdotoxin was subsequently shown to inhibit four different K+ channels (Kv1.3 and three distinct small conductance Ca++ activated K+ channels) in human T-lymphocytes, limiting the use of this toxin as a probe for the physiological role of Kv1.3 (Leonard et al., Proc. Natl, Acad. Sci. USA, 89, 10094, 1992). Margatoxin, on the other hand, blocks only Kv1.3 in T-cells, and has immunosuppressant activity on both in in vitro and in vivo models. (Lin et al., J. exp. Med, 177, 637, 1993). The therapeutic utility of this compound, however, is limited by its potent toxicity. Recently, a class of compounds has been reported that may be an attractive alternative to the above mentioned drugs, see for example U.S. Pat. Nos. 5,670,504; 5,631,282; 5,696,156; 5,679,705; and 5,696,156. While addressing some of the activity/toxicity problems of previous drugs, these compounds tend to be of large molecular weight and are generally produced by synthetic manipulation of a natural product, isolation of which is cumbersome and labor intensive.
Immunoregulatory abnormalities have been shown to exist in a wide variety of autoimmune and chronic inflammatory diseases, including systemic lupus erythematosis, chronic rheumatoid arthritis, type I and II diabetes mellitus, inflammatory bowel disease, biliary cirrhosis, uveitis, multiple sclerosis and other disorders such as Crohn's disease, ulcerative colitis, bullous pemphigoid, sarcoidosis, psoriasis, ichthyosis, Graves ophthalmopathy and asthma.
Although the underlying pathogenesis of each of these conditions may be quite different, they have in common the appearance of a variety of auto-antibodies and self-reactive lymphocytes. Such self-reactivity may be due, in part, to a loss of the homeostatic controls under which the normal immune system operates. Similarly, following a bone-marrow or an organ transplantation, the host lymphocytes recognize the foreign tissue antigens and begin to produce antibodies which lead to graft rejection.
One end result of an autoimmune or a rejection process is tissue destruction caused by inflammatory cells and the mediators they release. Anti-inflammatory agents such as NSAID's act principally by blocking the effect or secretion of these mediators but do nothing to modify the immunologic basis of the disease. On the other hand, cytotoxic agents, such as cyclophosphamide, act in such a nonspecific fashion that both the normal and autoimmune responses are shut off. Indeed, patients treated with such nonspecific immunosuppressive agents are as likely to succumb from infection as they are from their autoimmune disease.
Cyclosporin A(CsA), which was approved by the US FDA in 1983 is currently the leading drug used to prevent rejection of transplanted organs. In 1993, FK-506 (Prograf) was approved by the US FDA for the prevention of rejection in liver transplantation. CsA and FK-506 act by inhibiting the body's immune system from mobilizing its vast arsenal of natural protecting agents to reject the transplant's foreign protein. In 1994, CsA was approved by the US FDA for the treatment of severe psoriasis and has been approved by European regulatory agencies for the treatment of atopic dermatitis. Though they are effective in fighting transplant rejection, CsA and FK-506 are known to cause several undesirable side effects including nephrotoxicity, neurotoxicity, and gastrointestinal discomfort. Therefore, a selective immunosuppressant without these side effects still remains to be developed. Potassium channel inhibitors promise to be the solution to this problem.
Atrial fibrillation (AF) and atrial flutter are the most common cardiac arrhythmias in clinical practice and are likely to increase in prevalence with the aging of the population. Currently, AF affects more than 1 million Americans annually, represents over 5% of all admissions for cardiovascular diseases and causes more than 80,000 strokes each year in the United States. While AF is rarely a lethal arrhythmia, it is responsible for substantial morbidity and can lead to complications such as the development of congestive heart failure or thromboembolism. Currently available Class I and Class III antiarrhythmic drugs reduce the rate of recurrence of AF, but are of limited use because of a variety of potentially adverse effects including ventricular proarrhythmia. Because current therapy is inadequate and fraught with side effects, there is a clear need to develop new therapeutic approaches
Antiarrhythmic agents of Class III are drugs that cause a selective prolongation of the duration of the action potential without significant cardiac depression. Available drugs in this class are limited in number. Examples such as sotalol and amiodarone have been shown to possess interesting Class III properties (Singh B. N., Vaughan Williams E. M. “A Third Class of Anti-Arrhythmic Action: Effects On Atrial And Ventricular Intracellular Potentials And Other Pharmacological Actions On Cardiac Muscle, of MJ 1999 and AH 3747” Br. J. Pharmacol 1970; 39:675–689, and Singh B. N., Vaughan Williams E. M, “The Effect of Amiodarone, A New Anti-Anginal Drug, On Cardiac Muscle”, Br J. Pharmacol 1970; 39:657–667), but these are not selective Class III agents. Sotalol also possesses Class II effects which may cause cardiac depression and is contraindicated in certain susceptible patients. Amiodarone, also is not a selective Class III antiarrhythmic agent because it possesses multiple electrophysiological actions and is severely limited by side effects (Nademanee, K. “The Amiodarone Odessey”. J. Am. Coll. Cardiol. 1992;20:1063–1065.) Drugs of this class are expected to be effective in preventing ventricular fibrillation. Selective class III agents, by definition, are not considered to cause myocardial depression or an induction of arrhythmias due to inhibition of conduction of the action potential as seen with Class I antiarrhythmic agents.
Class III agents increase myocardial refractoriness via a prolongation of cardiac action potential duration. Theoretically, prolongation of the cardiac action potential can be achieved by enhancing inward currents (i.e. Na+ or Ca2+ currents; hereinafter INa and ICa, respectively) or by reducing outward repolarizing potassium (K+) currents. The delayed rectifier (IK) K+ current is the main outward current involved in the overall repolarization process during the action potential plateau, whereas the transient outward (Ito) and inward rectifier (IKI) K+ currents are responsible for the rapid initial and terminal phases of repolarization, respectively. Cellular electrophysiologic studies have demonstrated that IK consists of two pharmacologically and kinetically distinct K+ current subtypes, IKr (rapidly activating and deactivating) and IKs (slowly activating and deactivating) (Sanguinetti and Jurkiewicz, Two Components Of Cardiac Delayed Rectifier K+ Current: Differential Sensitivity To Block By Class III Antiarrhythmic Agents, J Gen Physiol 1990, 96:195–215). Class III antiarrhythmic agents currently in development, including d-sotalol, dofetilide (UK-68,798), almokalant (H234/09), E-4031 and methanesulfonamide-N-[1′-6-cyano-1,2,3,4-tetrahydro-2-naphthalenyl)-3,4-dihydro-4-hydroxyspiro[2H-1-benzopyran-2,4′-piperidin]-6yl]monochloride, predominantly, if not exclusively, block IKr. Although, amiodarone is a blocker of IKs (Balser J. R. Bennett, P. B., Hondeghem, L. M. and Roden, D. M. “Suppression Of Time-Dependent Outward Current In Guinea Pig Ventricular Myocytes: Actions Of Quinidine And Amiodarone. Circ. Res. 1991, 69:519–529), it also blocks INa and ICa, effects thyroid function, is as a nonspecific adrenergic blocker, and acts as an inhibitor of the enzyme phospholipase (Nademanee, K. “The Amiodarone Odessey”, J. Am. Coll. Cardiol. 1992; 20:1063–1065). Therefore its method of treating arrhythmia is uncertain. Most Class III agents that are known to be in development predominantly block IKr.
Reentrant excitation (reentry) has been shown to be a prominent mechanism underlying supraventricular arrhythmias in man. Reentrant excitation requires a critical balance between slow conduction velocity and sufficiently brief refractory periods to allow for the initiation and maintenance of multiple reentry circuits to coexist simultaneously and sustain AF. Increasing myocardial refractoriness by prolonging action potential duration (APD), prevents and/or terminates reentrant arrhythmias. Most selective Class III antiarrhythmic agents currently in development, such as d-sotalol and dofetilide predominantly, if not exclusively, block Ikr, the rapidly activating component of IK found both in the human atrium and ventricle.
Since these Ikr blockers increase APD and refractoriness both in atria and ventricle without affecting conduction per se, theoretically they represent potential useful agents for the treatment of arrhythmias like AF. These agents have a liability in that they have an enhanced risk of proarrhythmia at slow heart rates. For example, torsades de points has been observed when these compounds are utilized (Roden, D. M. “Current Status of Class III Antiarrhythmic Drug Therapy”, Am J. Cardiol, 1993; 72:44B–49B). This exaggerated effect at slow heart rates has been termed “reverse frequency-dependence”, and is in contrast to frequency-independent or frequency-dependent actions (Hondeghem, L. M. “Development of Class III Antiarrhythmic Agents”. J. Cadiovasc. Cardiol. 20 (Suppl.2):S17–S22).
The slowly activating component of the delayed rectifier (Iks) potentially overcomes some of the limitations of Ikr blockers associated with ventricular arrhythmias. Because of its slow activation kinetics however, the role of Iks in atrial repolarization may be limited due to the relatively short APD of the atrium. Consequently, although Iks blockers may provide distinct advantage in the case of ventricular arrhythmias, their ability to affect SVT is considered to be minimal.
The ultra-rapidly activating delayed rectifier K+ current (Ikur) is believed to represent the native counterpart to a cloned potassium channel designated Kv1.5 and, while present in human atrium, it appears to be absent in human ventricle. Furthermore, because of its rapidity of activation and limited slow inactivation, Ikur is believed to contribute significantly to repolarization in human atrium. Consequently, a specific blocker of Ikur, that is a compound which blocks Kv1.5, would overcome the short coming of other compounds by prolonging refractoriness by retarding repolarization in the human atrium without causing the delays in ventricular reporlarization that underlie arrhythmogenic after depolarizations and acquired long QT syndrome observed during treatment with current Class III drugs.
In intact human atrial myocytes an ultra-rapidly activating delayed rectifier K+ current Ikur which is also known as the sustained outward current, LSUS or ISO, has been identified and this current has properties and kinetics identical to those expressed by the human K+ channel clone (hKv1.5, HK2) when isolated from human heart and stably expressed in human (HEK-293) cell lines. (Wang et al., 1993, Circ Res 73:1061–1076; Fedida et al., 1993, Circ Res 73:210–216; Snyders et al., 1993, J Gen Physiol 101:513–543) and originally cloned from rat brain (Swanson et al., 10, Neuron 4:929–939). Although various antiarrythmic agents are now available on the market, those having both satisfactory efficacy and a high margin of safety have not been obtained. For example, antiarrythmic agents of Class I according to the classification scheme of Vaughan-Williams (“Classification Of Antiarrhythmic Drugs: In: Cardiac Arrhythmias, edited by: E. Sandoe, E. Flensted-Jensen, K. Olesen; Sweden, Astra, Sodertalje, pp449–472, 1981) which cause a selective inhibition of the maximum velocity of the upstroke of the action potential (max) are inadequate for preventing ventricular fibrillation. In addition, they have problems regarding safety, namely, they cause a depression of myocardial contractility and have a tendency to induce arrhythmias due to an inhibition of impulse conduction. Beta-adrenoceptor blockers and calcium antagonists which belong to Class II and IV, respectively, have a defect in that their effects are either limited to a certain type of arrhythmia or are contraindicated because of their cardiac depressant properties in certain patients with cardiovascular disease. Their safety, however, is higher than that of the antiarrhythmic agents of Class I.